In order to achieve very high density storage of information, memory devices have been constructed in the prior art which utilize ion beams to write information by producing radiation damage within a localized region of a semiconducting diode. The localized region is produced, for test purposes, by implantation of ions through a mask, although it could be produced by means of a focused ion beam. Information is read out of such a memory device by means of a focused electron beam. When the beam strikes a region of the semiconducting diode which has been damaged, the current which it generates in the diode is reduced, thereby producing a binary "1" signal at an address created by the x,y coordinates of the electron beam position on the diode surface. Because the electron beam is scattered by the material of the diode, this method suffers a fundamental limitation in regard to resolution, and hence in regard to the density of information storage. For example a finely focused beam of electrons of 30 keV energy is scattered into a nearly hemispherical volume of approximately one micron radius, with center at the point of impact of the beam on the diode surface. Although the size of this electron "bloom" can be reduced by using electrons of lower energy, its lateral dimension remains approximately equal to the maximum penetration depth of the electrons. The penetration depth must in turn be greater than the thickness of the surface layer of the diode, which is typically a fraction of a micron is extent. However if ions are used to produce the current in the diode, the multiple scattering is much reduced because of the large mass of the ions compared to electrons. The limits to resolution are smaller, and are set by such factors as the smallest region of radiation damage which can be produced and the lateral diffusion of holes and electrons as they travel across the depletion layer of the diode.
Means have also been described whereby ion beams may be used to write information. These means include thermal activation, mass removal by sputtering, and addition of new material. When used as proposed in combination with electron beam readout, these means suffer from the same fundamental limitation in regard to resolution as does displacement-type radiation damage.